1. Field of the Invention
The disclosure relates to a system and method for reducing vibrations on floating platforms for drilling and production. More particularly, the disclosure relates to a system and method to reduce vortex-induced vibrations for a floating platform, such as a spar offshore platform.
2. Description of the Related Art
Offshore oil and gas drilling and production operations typically involve a platform, sometimes called a rig, on which the drilling, production and storage equipment, together with the living quarters of the personnel manning the platform, if any, may be mounted. Floating offshore platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and may be held in position over the well site by, as examples, mooring lines anchored to the sea floor, motorized thrusters located on the sides of the platform, or both. Although floating offshore platforms may be more complex to operate because of their movement in response to environmental conditions, such as wind and water movement, they are generally capable of operating in substantially greater water depths than are fixed platforms. There are several different types of known floating platforms, such as, for example, so-called “drill ships,” tension-leg platforms (TLPs), semi-submersibles, and spar platforms.
Spar platforms, for example, comprise long, slender, buoyant hulls that give them the appearance of a column, or spar, when floating in an upright, operating position, in which an upper portion extends above the waterline and a lower portion is submerged below it. Because of their relatively slender, elongated shape, they have relatively deeper drafts, and hence, substantially better heave characteristics, e.g., much longer natural periods in heave, than other types of platforms. Accordingly, spar platforms have been thought by some as a relatively successful platform design over the years. Examples of spar-type floating platforms used for oil and gas exploration, drilling, production, storage, and gas flaring operations may be found in the patent literature in, e.g., U.S. Pat. No. 6,213,045 to Gaber; U.S. Pat. No. 5,443,330 to Copple; U.S. Pat. Nos. 5,197,826; 4,740,109 to Horton; U.S. Pat. No. 4,702,321 to Horton; U.S. Pat. No. 4,630,968 to Berthet et al.; U.S. Pat. No. 4,234,270 to Gjerde et al.; U.S. Pat. No. 3,510,892 to Monnereau et al.; and U.S. Pat. No. 3,360,810 to Busking.
While spar offshore platforms are inherently less prone to heave because of their length, improvements in heave and motion control have been made by attaching horizontally disposed plates to the bottom of the spar hull and at times radially extending plates around the circumference of the hull. The horizontal plates have a significant width and length in an X-Y axis and a relatively small height in a Z-axis orthogonal coordinate system with the Z-axis being vertical along the length of the spar platform, as the spar is normally disposed during offshore use. U.S. Pat. No. 3,500,783 to Johnson, et al., discloses radially extending fins from the hull with a heave plate at the bottom of the hull, in that vertically and radially extending damping plates are circumferentially spaced around the upper and lower submerged portions of the platform and a horizontal damping plate is secured to the bottom of the platform to prevent resonance oscillation of the platform. Further improvements to heave control of the spar have been made by extending the spar length with open structures below the hull, such as trusses, and installing horizontally disposed plates in the open structures. The open structure of the truss allows water to be disposed above and below the surface of the horizontal plate, so that the water helps dampen the vertical movement of the spar platform.
Despite their relative success, current designs for spar platforms offer room for improvement. For example, because of their elongated, slender shape, they can be relatively more complex to manage during offshore operations under some conditions than other types of platforms in terms of, for example, control over their trim and stability. In particular, because of their elongated, slender shape, spar platforms may be particularly susceptible to vortex-induced vibration (VIV) or vortex induced motion (VIM) (herein collectively, “VIV”), which may result from strong water currents acting on the hull of the platform.
More specifically, VIV is a motion induced on bodies facing an external flow by periodical irregularities of this flow. Fluids present some viscosity, and fluid flow around a body, such as a cylinder in water, will be slowed down while in contact with its surface, forming a boundary layer. At some point, this boundary layer can separate from the body. Vortices are then formed, changing the pressure distribution along the surface. When the vortices are not formed symmetrically around the body with respect to its midplane, different lift forces develop on each side of the body, thus leading to motion transverse to the flow. VIV is an important cause of fatigue damage of offshore oil exploration and production platforms, risers, and other structures. These structures experience both current flow and top-end vessel motions, which give rise to the flow-structure relative motion. The relative motion can cause VIV “lock-in”. “Lock-in” occurs when the reduced velocity, Urn, is in a critical range depending on flow conditions and can be represented according to the formula below:1<Ur=uTn/D<12 where:                Ur: Reduced velocity based on natural period of the moored floating structure        u: Velocity of fluid currents (meters per second)        Tn: Natural period of the floating structure in calm water without current (seconds)        D: Diameter or width of column (meters)        
Stated differently, lock-in can occur when the vortex shedding frequency becomes close to a natural frequency of vibration of the structure. When lock-in occurs, large-scale, damaging vibrations can result.
The typical solution to VIV on a spar platform is to provide strakes along the outer perimeter of the hull. The strakes are typically segmented, helically disposed structures that extend radially outward from the hull in two or more lines around the hull. Strakes have been effective in reducing the VIV. Examples are U.S. Pat. No. 6,148,751 to Brown et al., for a “system for reducing hydrodynamic drag and VIV” for fluid-submersed hulls, and U.S. Pat. No. 6,244,785, to Richter et al., for a “precast, modular spar system having a cylindrical open-ended spar.” Further, U.S. Pat. No. 6,953,308 to Horton discloses strakes that radially extend from the hull and radially extending horizontal heave plates. A significant improvement in strake design is shown in WO 2010/030342 A2 for a spar hull that includes a folding strake that can be deployed for example at installation. However, strakes can be labor intensive, and difficult to install and transport undamaged to an installation site of the spar platform.
A different alleged solution to vortex induced forces and motion is disclosed in U.S. Publ. No. 2009/0114002 where surface roughness on a bluff body decreases vortex induced forces and motion, and can be applied to flexible or rigid cylinders, such as underwater pipelines, marine risers, and spar offshore platforms.
There remains a need for improved and more efficient reduction in VIV for floating platforms.